Mentally ill offenders constitute a group with a unique set of characteristics since they are doubly stigmatized by both their mental illness and the offence they have committed. The coexistence of these two circumstances significantly heightens negative public attitudes towards these people. The group of mentally ill offenders has been shown to elicit more stigmatic attitudes than offenders without a mental health condition. Nevertheless, research on stigma towards mentally ill offenders is rather limited, while the number of psychometric tools used to measure this stigma is even smaller compared to the number of relevant tools assessing mental illness stigma. The purpose of this study was to explore the attitudes towards mentally ill offenders in a Greek sample in terms of demographic characteristics, and at the same time to assess the psychometric properties of a specialized tool on stigma towards this patient group, namely the Attitudes Towards Mentally Ill Offenders (ΑΤΜΙΟ) scale in Greek. The study included 1031 participants from the general population who completed an online questionnaire on sociodemographic data as well as the ATMIO scale. The scale's structural validity was tested on the basis of the exploratory factor analysis after Quartimax rotation, and the internal relevance of its factors recorded a Cronbach's alpha value of more than 0.7, both for the whole scale and its individual factors. It was shown that more negative stereotypes towards mentally ill offenders were correlated with less compassion and less desire for their rehabilitation, with stronger belief and conviction that they represent a danger to the community, with less diminished responsibility, and a lot fewer positive attitudes in general. Women, older people, individuals with a lower education level and participants with children were found to hold more negative attitudes. The ATMIO scale translated in Greek is the first tool to measure attitudes towards mentally ill offenders in the country and shows satisfactory internal consistency and interpretation of its four-factor structure. It is a comprehensible and easy-to-complete scale, which can become a reliable tool to record attitudes towards mentally ill offenders also in our country.